1. Field of the Invention
The present invention relates to an injection foaming mold, a method of producing a foamed plastic product, and a foamed plastic product, and more particularly, to an injection foaming mold to be used for a core back, a method of producing a foamed plastic product, and a formed resin molding product.
2. Description of the Related Art
Conventionally, a foamable resin is produced by introducing a foaming agent such as butane gas, methane gas, water, nitrogen, or carbon dioxide to a resin or by melt-kneading the resin and the foaming agent mechanically, or by inducing a chemical reaction therebetween.
Then, the foamable resin thus produced is injected or extruded to a die by an injection molding machine or an extrusion molding machine to produce a foamed plastic product with a desired shape and a foaming ratio.
Further, in recent years, as a method of producing a foamed plastic product, there has been known a method of obtaining a formed resin molding product containing a large number of cells with a small (micro size) foaming diameter by introducing nitrogen or carbon dioxide in a supercritical state to a resin material at high pressure and high temperature, and adjusting the pressure and the temperature (for example, U.S. Pat. No. 4,473,665).
Further, as a method of producing a foamed plastic product, there is known a foam molding method called core back (for example, Japanese Publication No. S39-022213 (of examined patent application)).
According to the method using the core back, after a foamable resin is injected to a die, under a state in which the foamable resin is uncured, the capacity of the die is enlarged to decrease the pressure applied to the resin in an uncured state. Then, gas contained in the resin in an uncured state is formed into cells to increase a foaming ratio.
Further, as a foam molding method using the core back, there is known a molding method in which the core back is performed partially, in addition to a method in which the core back is performed entirely (for example, Japanese Patent Application Laid-Open No. 2003-170762).
The conventional foam molding has a problem in that, when a foaming diameter is increased so as to enhance a foaming ratio as in the case of foam polystyrene used in a packing material, partition walls between foam cells become thin to decrease the strength of a molding product remarkably.
Further, according to the above-mentioned foam molding method using inactive gas in a supercritical state, which is described in U.S. Pat. No. 4,473,665, the decrease in strength of the molding product is small because the cell diameter has a micro size.
However, it is difficult to enhance a foaming ratio, and the reduction in weight by foaming stands at about 10%. Thus, there is a problem in that the economic effect such as the reduction in materials is low.
In contrast, according to the above-mentioned foam molding methods using the core back described in Japanese Publication No. S39-022213 (of examined patent application) and Japanese Patent Application Laid-Open No. 2003-170762, the reduction in weight can be achieved while a foaming ratio is enhanced without decreasing the strength such as stiffness.
However, according to the foam molding method using the core back, particularly in the case of performing the core back partially, a shrinkage ratio varies due to the difference in foaming ratio in the vicinity of a boundary in portions in which the die capacity is enlarged and at which the die capacity is not enlarged, or in portions in which a partial difference in enlargement ratio occurs.
Therefore, there is a problem in that a step difference and deformation similar to a sink are generated, degrading an outer appearance and component precision.
Further, there is a problem in that, after performing the core back, a portion that is not subjected to the core back is swollen when being taken out of the die.
Such swelling is caused because it is necessary to increase a force for foaming so as to enhance the foaming ratio by the core back.
That is, such swelling is caused in such a manner that a portion subjected to the core back is released from a force by the reduction in pressure by performing the core back so as to enhance a foaming ratio, whereas a portion that is not subjected to the core back is not reduced in pressure and a force for foaming remains in that portion as it is.